Optical pickup and optical disc apparatus

ABSTRACT

When recording/reproducing an optical disc having a recording layer of multi-layer structure, an unwanted optical beam reflected from a recording layer other than a target layer for recording/reproduction is incident on a photodetector to cause an unwanted disturbance component to leak to a detection signal, giving rise to a degradation in the quality of a tracking control signal. In an optical pickup apparatus, for suppression of the degradation, an optical element is mounted having a diffraction area for diffracting part of the optical beam and light receiving planes for sub-optical beams are provided each of which has a light shielding zone or dead zone of a predetermined width on its central sectioning line.

CLAIM OF PRIORITY

This application is a continuation of application Ser. No. 11/734,817,filed on Apr. 13, 2007, now pending, which claims the benefit ofJapanese Application Nos. JP2006-112893 filed on Apr. 17, 2006 andJP2007-029929 filed on Feb. 9, 2007 in the Japanese Patent Office, thedisclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an optical pickup and an optical discapparatus mounting the same and an optical information recording andreproducing apparatus as well.

As a background art concerning the present technology, JP-A-7-272303(Patent Document 1), for example, is available. The public literaturedescribes an object reading “providing an optical disc apparatus and anoptical pickup which can make the overall size compact and simplify theassembling work by virtue of a simplified construction and a method ofassembling the optical pickup as well” and describes a solution reading“addition of output signals from light receiving planes E and G andaddition of output signals from light receiving planes F and H areprecedently carried out in advance amplification circuits 15E and 15Hand the resulting sum signals are amplified and delivered”.

Further, available as another background art concerning the field of thepresent technology is, for example, JP-A-2005-203090 (Patent Document2). The public literature describes an object reading “providing anoptical pickup apparatus which can suppress, during recording and/orreproduction of an optical disc having a plurality of recording layerson one side surface, interference light caused by adjoining layers tothereby eliminate fluctuations in a tracking error signal detectedthrough DPP (difference push-pull) and describes a solution reading “anoptical member is provided which can, when applied to an opticalinformation storage medium having a plurality of recording layers on atleast one surface, restrain interference light caused by adjoininglayers from being received by a photodetector. Through this, theinterference light due to adjoining layers can be restrained from beingreceived by the photodetector, especially, by first and secondsub-detectors of the photodetector.

SUMMARY OF THE INVENTION

As a contrivance for realizing a large capacity of optical discrecording, a technology concerning an optical information recordingmedium having a recording layer of multi-layer structure (hereinafter,simply referred to as an optical disc) has recently been developed.

Further, so-called tracking control is used for focusing a laser beamstably and accurately on a predetermined recording track formed on therecording layer of the optical disc. Then, as a tracking control signaldetection scheme, the differential push-pull scheme (DPP scheme)generally comes up.

Recently, in the course of recording/reproduction of an optical dischaving a recording layer of multi-layer structure, a problem that thetracking control signal detected through the DPP scheme changes owing tointerlayer cross talk has been raised.

From the standpoint of geometrical optics, the solution described inPatent Document 2 succeeds in suppressing the change in the through theDPP scheme detected tracking control signal attributable to theinterlayer cross talk at the time of recording/reproduction of theoptical disc having a recording layer of multi-layer structure. But, inspite of the fact that the solution described in Patent Document 2 iseffective to keep unwanted light from the adjoining layer from landingon the photodetector, an interlayer cross talk still occurs in effect,raising a problem that generation of a highly accurate and stabletracking control signal is difficult to achieve.

An object of the present invention is to provide an optical pickup andan optical information recording/reproduction apparatus which canoperate stably and highly precisely even for a recording medium having arecording layer of multi-layer structure.

Another object of this invention is to provide an optical pickupapparatus and an optical disc apparatus which can obtain a highlyaccurate and stable tracking control signal.

The above objects can be accomplished by the constitution as recited inappended claims.

According to the present invention, stable and highly precise opticalpickup and optical information recording/reproduction apparatus can beprovided.

In addition, according to the present invention, optical pickupapparatus and optical disc apparatus capable of obtaining a highlyaccurate and stable tracking control signal can be provided.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view diagram showing a photodetectorrepresenting a main part in a first embodiment.

FIG. 2 is a schematic front view diagram showing an optical systemconfiguration of an optical pickup in the conventional technologies andin the present invention.

FIG. 3 is a schematic plan view diagram showing a conventional exampleof photodetector.

FIG. 4 is a schematic sectional diagram of an optical disc having amulti-layer structure.

FIGS. 5A and 5B are schematic sectional diagrams showing optical pathsof optical beams incident on the multi-layer optical disc, respectively.

FIG. 6 is a schematic plan view diagram showing a photodetectorrepresenting a main part in a second embodiment.

FIG. 7 is a schematic front view showing the shape of a diffractiongrating used in a third embodiment.

FIG. 8 is a schematic plan view diagram showing the state of sub-opticalbeams incident on the photodetector in the third embodiment.

FIG. 9 is a block diagram of an optical informationrecording/reproduction apparatus mounting the optical pickup accordingto first to fifth embodiments.

FIG. 10 is a schematic plan view diagram showing a photodetectorrepresenting a main part in a fourth embodiment.

FIG. 11 is a schematic plan view diagram showing a photodetectorrepresenting a main part in a fifth embodiment.

FIG. 12 is a schematic diagram showing an optical system configurationof an optical pickup apparatus in the present invention.

FIG. 13 is a schematic plan view diagram showing a conventional exampleof the photodetector.

FIGS. 14A and 14B are schematic sectional diagrams showing optical pathsof optical beams incident on an optical disc of multi-layer structure,respectively.

FIG. 15 is a schematic diagram showing an example of the shape of adiffraction area an optical element for diffracting part of an opticalbeam has.

FIGS. 16A and 16B are schematic diagrams showing light intensitydistributions of signal optical beam and unwanted optical beams on thephotodetector, respectively, when the optical element of FIG. 15 ismounted.

FIG. 17 is a schematic plan view diagram showing a photodetectorrepresenting a main part in a sixth embodiment.

FIG. 18 is a graph showing results of simulation of amounts ofinterlayer cross talk leakage to a sub-PP signal in relation topositional shifts of the light receiving plane in the sixth embodiment.

FIGS. 19A and 19B are schematic plan view diagrams showing aphotodetector representing a main part in a seventh embodiment.

FIG. 20 is a schematic diagram showing a light intensity distribution ofa signal optical beam on the photodetector when an objective lens isshifted.

FIG. 21 is a graph showing simulation results of amounts of de-trackgeneration in relation to the objective lens shift in the sixth andseventh embodiments.

FIG. 22 is a schematic plan view diagram showing a photodetectorrepresenting a main part in an eighth embodiment.

FIG. 23 is a schematic plan view diagram showing a photodetectorrepresenting a main part in a ninth embodiment.

FIG. 24 is a schematic diagram showing the shape of a diffractiongrating representing a main part in a tenth embodiment.

FIG. 25 is a schematic plan view diagram showing a photodetector and theshape of a spot of individual optical beams in the tenth embodiment.

FIG. 26 is a schematic diagram showing an example of an optical discapparatus mounting the optical pickup apparatus according to theinvention.

DESCRIPTION OF THE EMBODIMENTS

Details of embodiments of the present invention will be describedhereunder with reference to the accompanying drawings. It will beappreciated that in individual drawings, like constituent componentsfunctioning identically are designated by like reference numerals.

Embodiment 1

The DPP scheme will be described in brief.

Referring to FIG. 2, a main optical system of an optical pickup using atracking control signal detection means based on the DPP scheme isconfigured as schematically illustrated therein.

A laser beam generated from a semiconductor laser 1 is split by a beamsplitting element such as diffraction grating 2 into a main optical beam50 (o-th order) for actual reproduction or recording of an informationsignal and two sub-optical beams 51 and 52 (positive and negative firstorder diffracted beams). These optical beams travel through half mirror3 and collimate lens 4 and they are focused independently on apredetermined recording layer inside an optical disc 10 by means of anobjective lens 5. At that time, a focused spot of main optical beam 50(not shown) and focused spots of sub-optical beams 51 and 52 (not shown)are irradiated on the optical disc 10 at such positions that they arespaced apart in the radial direction of the optical disc 10 at intervalsequal to substantially half the recording track spacing of the opticaldisc 10. Then, these focused light spots are reflected by the opticaldisc 10 to generate reflection optical beams which return through theobjective lens 5, collimate lens 4 and half mirror 3 and then land on aphotodetector 8 by way of a detection lens 7.

The objective lens 5 is attached with an actuator 6 for driving the samein a predetermined direction and a tracking control signal to bedescribed later is fed back to the actuator 6 to control the position ofthe objective lens, thus executing tracking control.

Incidentally, the photodetector 8 includes, as shown in FIG. 3, a lightreceiving area 80 on which a focused spot 60 originating from theoptical disc reflected light of main optical beam 50 is incident andlight receiving areas 81 and 82 on which light spots 61 and 62originating from the optical disc reflected light of sub-optical beams51 and 52, respectively, are incident, the areas 81 and 82 beingdisposed in parallel above and below the area 80 on the sheet ofdrawing. Of these areas, the light receiving area 80 for optical beam 50forms, for example, a light receiving plane divided into four divisionalareas by a cruciform sectioning line as shown in the figure and on theother hand, each of the light receiving areas 81 and 82 for sub-opticalbeams forms a light receiving plane which is halved vertically in thefigure by a sectioning line 83 or 84. Then, currents are generated fromthe individual divisional light receiving planes in accordance withintensities of light incident thereon and these currents are convertedinto voltages independently of one another by means of current-voltageconversion amplifiers 201 to 208 and mutually subtracted by means ofsubtractors 210 to 211 so that a push-pull signal of the main opticalbeam 50 (for simplicity of description, referred to as a main PP signal)and a sum signal of push-pull signals of the respective sub-opticalbeams 51 and 52 (for simplicity of description, referred to as a sub-PPsignal) may be outputted.

Since the respective focused spots incident on the optical disc 10 arearrayed as described above, the main PP signal and the sub-PP signal nowdelivered are 180° dephased with each other. Therefore, it is socontrived that by amplifying the both types of PP signals with suitablemu-factors K1 and K2 by means of amplifiers 212 and 213, respectively,and thereafter subtracting them from each other by means of a subtractor214, an excellent tracking control signal can be outputted which isremoved of unwanted DC components and in-phase disturbance componentscontained in both the main PP signal and sub-PP signal.

As will be seen from the above, the DPP scheme has an advantage that anoffset or the like of tracking control signal caused concomitantly with,for example, a tracking displacement of the objective lens can beeliminated with the simplified optical system configuration and atracking control signal of high quality can be detected stably, thusenjoying a widespread use for a tracking control signal detection means.An operation circuit constructed of subtractors 210 and 211, amplifiers212 and 213 and subtractor 214 to detect the tracking control signal asdescribed above will hereinafter be termed a tracking control signaldetection circuit 500.

Controlling the position of objective lens 5 in optical pickup is notlimited to only the aforementioned tracking control but obviously,position control along the optical axis direction, so-called focuscontrol, is performed concurrently. Then, for detection of a focuscontrol signal used in the focus control, an astigmatism scheme, forexample, is used in general and like the tracking control signal, thefocus control signal can be generated from signals detected at theindividual light receiving planes of the photodetector 8 shown in FIG.3, for example, by way of a predetermined operation circuit.

In this manner, the DPP scheme has been used widely.

But when the tracking control signal detection means based on the DPPscheme as above is used for an optical pickup or optical informationrecording/reproduction apparatus adapted to reproduce or record anoptical disc having a recording layer of multi-layer structure, thefollowing problem arises newly.

More particularly, when respective optical beams are focused on arecording layer undergoing actual recording or reproduction of a signal(hereinafter simply referred to as a target layer) among the individualrecording layers in the multi-layer optical disc, part of the quantityof light is not reflected at the target layer but is reflected by arecording layer other than the target layer, disadvantageously resultingin an unwanted optical beam failing to contribute to actual signaldetection, which unwanted optical beam traces substantially the sameoptical path as that of the essential signal beam to land on theindividual light receiving planes in the photodetector. The unwantedoptical beam thus being incident on the light receiving plane interfereswith the essential signal beam on the light receiving plane to form aninterference fringe by which the quantity of light is unbalanced tocause an unwanted disturbance component to leak to the signal deliveredout of each of the light receiving planes.

This phenomenon will be described concretely by taking an optical disc10 having a recording layer structure of two layers 100 and 101 (layerspacing δ) as shown in FIG. 4, for instance.

On the optical disc 10 having the two recording layers 100 and 101 asshown in FIG. 4, a main optical beam 50 and sub-optical beams 51 and 52(not shown) are focused from below in the figure as schematicallyillustrated, for example, in FIGS. 5A and 5B.

Firstly, illustrated in FIG. 5A is an instance where the respectiveoptical beams are focused on a forehand (lower in the figure) recordinglayer 100, that is, the recording layer 100 is a target layer. In thiscase, part of the light quantity of a light spot focused on therecording layer 100 transmits through the recording layer 100 and isthen reflected at an inner (upper in the figure) recording layer 101 tocause an unwanted optical beam 53.

Contrary to the instance of FIG. 5A, the respective optical beams arefocused on the inner (upper in the figure) recording layer 101 in aninstance of FIG. 5B, that is, the recording layer 101 is a target layerin this instance. In this case, the optical beam once transmits throughthe forehand (lower in the figure) recording layer 100 and is thereafterfocused on the recording layer 101 while part of the light quantitybeing reflected at the recording layer 100 to cause an unwanted opticalbeam 54.

Any of the unwanted optical beams 53 and 54 traces substantially thesame optical path as that of the essential signal beam and reaches thephotodetector, greatly diverging to irradiate each of the lightreceiving planes in the photodetector. Then, part of the beam overlapsand interferes with the essential signal optical beam irradiated on eachlight receiving area. As a result, a light and dark interference fringeis generated on each light receiving area and the light quantity islocally unbalanced by the interference fringe, giving rise to anunwanted disturbance component which leaks to the signal detected fromeach of the light receiving planes.

Especially, the sub-PP signal used for tracking control signal detectionbased on the DPP scheme is generally smaller in signal intensity thanthe main PP signal and is therefore greatly affected by the lightquantity unbalance due to the aforementioned interference of theunwanted optical beam with the signal optical beam, so that a relativelylarge disturbance component as compared to the actual signal amplitudeleaks to the sub-PP signal. Consequently, the tracking control signaldetected through the DPP scheme is extravagantly distorted in waveformand fluctuated, leading to degraded signal quality.

The present inventors have studied the degree of an influence theinterference of the unwanted optical beam with the signal beam has uponthe sub-PP signal to find that, of the light quantity unbalance in eachlight receiving area caused by the interference, a light quantityunbalance developing on and near the sectioning line (83 or 84 in FIG.3) formed in each of the light receiving areas for sub-PP signaldetection (light receiving areas 81 and 82 in FIG. 3) most adverselyaffects the quality of the sub-PP signal.

On the other hand, the main PP-signal and sub-PP signal per se aremainly generated by changes in light quantity the individual light spots(60 to 62 in FIG. 3) focused on the respective light receiving areasundergo at their peripheral edges and it has been known that an opticaldisc for which the information signal recording density is higher andconcomitantly the recording track spacing is set to be narrower tends tobe largely vulnerable to the adverse influence.

In the light of the circumstances as above, according to the presentinvention, in an optical pickup using a tracking control signaldetection means based on the DPP scheme and having the function torecord information signal on an optical disc having a multi-layerstructure of two or more layers or to reproduce the recorded informationsignal and an optical information reproducing apparatus mounting theoptical pickup, the degradation in quality of a tracking control signalcaused when an unwanted optical beam generated from a recording layerother than a target layer interferes with an essential signal opticalbeam on each of the light receiving areas of a photodetector can becured remarkably to assure detection of a stable and highly accuratetracking control signal.

A photodetector representing a principal part in the first embodiment isconstructed as shown in FIG. 1. In FIG. 1, the same constituentcomponents as those of the photodetector which has already beendescribed in connection with FIG. 3 are designated by the same referencenumerals.

An optical system configuration of an optical pickup in the presentembodiment may resemble that shown in connection with FIG. 2, forinstance, with the only exception that a light receiving plane patternin the photodetector 8 differs. Comparison of the light receiving planepattern of photodetector 8 shown in FIG. 1 according to the presentembodiment with the light receiving plane pattern of photodetector 8shown in FIG. 3 clarifies that in the case of the present embodiment,light shielding zones or dead zones 73 and 74 each having a minor sidewidth W set to a predetermined dimension as will be described later arerespectively formed on central sectioning lines (corresponding to thesectioning lines 83 and 84 in the conventional example in FIG. 3) oflight receiving areas 81 and 82 on which the focused spots 61 and 62 ofsub-optical beams are incident, respectively. The light shielding zonecan be realized by vapor-depositing a medium opaque to light such as forexample aluminum on the light receiving plane, thereby ensuring thatoptical beam can be prevented from directly reaching a light receivingplane portion covered with the light shielding zone. Thus, the presentembodiment can be applied to usual optical pickups in a simplifiedmanner. The dead zone can be materialized by, for example, partiallyremoving the light receiving plane at the counterpart portion so thateven when an optical beam actually lands on the portion, no responsivesignal current may be generated.

The light shielding zone is not limited to the medium such as aluminumas above exhibiting a transmission factor of substantially zero to theall wavelength band but may be a light shielding zone of a wavelengthselectable medium exhibiting a transmission factor of substantially zeroto a specified wavelength band, for example.

With the structure as above, the optical pickup corresponding to thepresent embodiment can be produced at low costs. Even when an unwantedoptical beam generated from a recording layer other than a reproductionor recording target layer interferes with an essential signal opticalbeam obtained from the recording/reproduction target layer in themulti-layer disc as described previously, a disturbance component sogenerated by the interference as to leak to the sub-PP signal can bereduced efficiently by providing the light shielding zone or dead zoneon each of the light receiving areas 81 and 82 for sub-optical beams.

On the other hand, the sub-PP signal per se is mainly generated by achange in light quantity at the peripheral edge of each light spotfocused on each light receiving area as described previously and istherefore hardly affected by the light shielding zone or dead zone. Inconsequence, even in the optical disc having the recording layer of amulti-layer structure, the tracking control signal based on the DPPscheme can be detected highly accurately and stably.

The study result by the present inventors proved that the width W of theminor side the light shielding zone or dead zone has can be set within arange of about 20% to 40% of the diameter each of the focused spots 61and 62 of sub-optical beams incident on the light receiving areas 81 and82 has in order to suppress the disturbance component most efficiently.More preferably, since the diameter each of the focused spots 61 and 62of sub-optical beams incident on the light receiving areas 81 and 82 hasis in most general set to about 100 μm in the ordinary optical pickup,the width W of the light shielding zone or dead zone minor side is setwithin a range of about 20 μm to 40 μm.

Embodiment 2

Next, a second embodiment will be described by making reference to FIG.6.

A principal part of the second embodiment is illustrated in FIG. 6. Inthe figure, too, the same constituent components as those shown in FIGS.1 and 3 are designated by the same reference numerals.

In the present embodiment, as substitution for the light shielding zoneor dead zone in the first embodiment shown in FIG. 1, sectioning lines85 and 86 substantially parallel to the original sectioning line 83 arenewly provided above and below it in the light receiving area 81 forsub-optical beam and sectioning lines 87 and 88 substantially parallelto the original sectioning line 84 are newly provided above and below itin the light receiving area 82 for sub-optical beam, so that the lightreceiving area 81 can be divided into four light receiving planes 81 athrough 81 d by the three sectioning lines and the light receiving area82 can be divided into four light receiving planes 82 a through 82 d bythe three sectioning lines. In this structure, the spacing W between thesectioning lines 85 and 86 and the spacing W between the sectioninglines 87 and 88 are each dimensionally comparable to the width W of theminor side the light shielding zone or dead zone has in the firstembodiment shown in FIG. 1.

For example, of the four divisional light receiving planes, the lightreceiving plane 81 a outputs a signal to a signal line 301 via acurrent/voltage conversion amplifier 201 and the light receiving plane81 d outputs a signal to a signal line 304 via a current/voltageconversion amplifier 202 and then from these signals, a sub-PP signal isgenerated which is exactly comparable to the sub-PP signal obtained fromthe photodetector of the first embodiment shown in FIG. 1. Quitesimilarly, the light receiving plane 82 a outputs a signal to a signalline 305 via a current/voltage conversion amplifier 207 and the lightreceiving plane 82 d outputs a signal to a signal line 308 via acurrent/voltage conversion amplifier 208 and then from these signals, asub-PP signal is generated which is exactly comparable to the sub-PPsignal obtained from the photodetector of the first embodiment shown inFIG. 1.

On the other hand, a signal delivered out of the light receiving plane81 a or 82 a and detected via the current/voltage conversion amplifier201 or 207 is added to a signal delivered out of the light receivingplane 81 b or 82 b and detected via current/voltage conversion amplifier209 or 211 by means of an adder 215 or 217 so as to provide a signaloutputted from signal line 302 or 306 and similarly, a signal deliveredout of the light receiving plane 81 d or 82 d and detected via thecurrent/voltage conversion amplifier 202 or 208 is added to a signaldelivered out of the light receiving plane 81 c or 82 c and detected viacurrent/voltage conversion amplifier 210 or 212 by means of an adder 216or 218 so as to provide a signal outputted from signal line 303 or 307and then from these output signals, a sub-PP signal is generated whichis exactly comparable to the sub-PP signal obtained from thephotodetector of the first embodiment shown in FIG. 3.

Then, in the present embodiment, transfer switches 401 and 402 are usedto selectively switch over the signal lines for generation of the sub-PPsignal in the manner described above to permit a single photodetector toplay the dual function of the photodetector of the present invention andthe conventional photodetector. Accordingly, through proper use of theaforementioned dual function depending on the kind of optical disc, forexample, the multi-layer disc or conventional single layer recordingdisc, the general use capability of the optical pickup can be improved.

Embodiment 3

Next, a third embodiment will be described with reference to FIGS. 7 and8. In the present embodiment, the optical system of optical pickup andphotodetector 8 are constructed substantially identically to those inFIGS. 2 and 3, with the only exception that a diffraction grating 2 usedas the beam splitting element is structured differently from theconventional one. The structure of a diffraction grating 2 used in thepresent embodiment is illustrated in schematic plan view form in FIG. 7.In the present embodiment, the diffraction grating 2 is divided intothree areas 21, 22 and 23 by means of two sectioning lines extending ina direction (corresponding to a tangential direction of the disc asviewed on the diffraction grating 2) which is substantially orthogonalto a grating groove direction (corresponding to a radial direction ofthe disc as viewed on the diffraction grating 2). Of these areas, twoareas 21 and 23 are grooved but the central area 22 sandwiched betweenthem is transparent and flat.

When a beam emitted from a laser light source lands on the diffractiongrating 2 of the structure as above, only part of the beam passingthough the central area 22 is not diffracted and a sub-optical beamdiffracted and separated from a main optical beam has a substantiallystrip-like blank at the central portion alone. Therefore, focused lightspots of the sub-optical beams finally landing on the light receivingareas 81 and 82 in the photodetector 8 by way of the optical disc areblank or removed at their strip-like portions which are expected toreach exactly on the sectioning lines 83 and 84 and their neighborhoodsas shown in FIG. 8, giving rise to focused spots each halved into spotportions 61 a and 61 b or spot portions 62 a and 62 b incident on eachlight receiving plane.

Accordingly, even with the light receiving area of photodetector 8structurally lacking the light shielding zone or dead zone, meritoriouseffects similar to those in the first embodiment shown in FIG. 1 can beobtained.

Generally, the trisectional diffraction grating structure as in thepresent embodiment is advantageous from the standpoint of cost and workefficiency over the provision of the light shielding zone or dead zoneon such a highly precise and expensive parts as the photodetector.

Preferably, the width W′ of central area 22 of the trisectional gratingshown in FIG. 7 is so designed that the width W between the focusedspots 61 a and 61 b on the light receiving area 81 of photodetector 8 orbetween the focused spots 62 a and 62 b on the light receiving area 82equal the width W of the light shielding zone or dead zone in the FIG. 1embodiment.

Embodiment 4

Next, a fourth embodiment of the present invention will be describedwith reference to FIG. 10. In the present embodiment, like the firstembodiment shown in FIG. 1, predetermined light shielding zones or deadzones 73 and 74 are provided on central sectioning lines of lightreceiving areas 81 and 82 for sub-optical beams, respectively, andbesides a sectioning line substantially orthogonal to the centralsectioning line is formed in each of the light receiving areas 81 and82, enabling each light receiving area to be quartered like the lightreceiving area 80 for main optical beam. Quartering each of the lightreceiving areas 81 and 82 for sub-optical beams additionally to thelight receiving area 80 for main optical beam has the aim of ensuringthat the focus control signal based on the astigmatism scheme can bedetected even in the light receiving areas 81 and 82 for sub-opticalbeam as in the case of the light receiving area 80 for main opticalbeam. In order that the focus control signal based on the astigmatismscheme can be detected from each of the light receiving area 80 for mainoptical beam and the light receiving areas 81 and 82 for sub-opticalbeams, an arithmetic operation circuit comprised of adders 228 through235 and subtractors 236 and 237 which are provided in a focus controlsignal detection circuit 501 is used as shown in FIG. 10 but detectionof a focus error signal based on the astigmatism scheme is of well-knowntechnology and will not be detailed herein.

The focus control signal detection circuit 501 includes, in addition tothe adders and subtractors, an amplifier 238 for amplifying, with apredetermined mu-factor K3, the focus control signal detected by way ofthe light receiving areas 81 and 82 for sub-optical beams and an adder239 for adding together the amplified sub-optical beam focus controlsignal and a focus control signal for main optical beam detected throughthe light receiving area 80 for main optical beam. The scheme for makingthe sum signal of focus control signals for main optical beam andsub-optical beam a new focus control signal is called a differentialastigmatic detection scheme (DAD scheme) and is effective to eliminate adisturbance component leaking to the focus control signal based on theastigmatism scheme and detect an excellent focus control signal. Thescheme per se belongs to a well-known technology and will not bedetailed herein.

Additionally provided in the focus control signal detection circuit 501is a transfer switch 403 for switching over from the aforementioned sumsignal of the focus control signal for main optical beam and the focuscontrol signal for sub-optical beam to the conventional focus controlsignal generated from only the main optical beam and vice versa so thatselective switch over to delivery of either focus control signal may becarried out depending on the kind of the optical disc subject toreproduction or recording.

Putting the focus control signal detection circuit 501 aside, a trackingcontrol signal detection circuit 500 similar to that in the FIGS. 1 and6 embodiments is included in the present embodiment but details of thetracking control signal detection circuit have already been given inconnection with FIGS. 1, 3 and 6 and will not be given herein.

Embodiment 5

A fifth embodiment of the present invention will now be described withreference to FIG. 11. In the present embodiment, the light receivingplane structure is substantially the same as that in the secondembodiment of the invention shown in FIG. 6 with the only exception thata sectioning line orthogonal to sectioning lines 83, 85 and 86 isprovided in a light receiving area 81 for sub-optical beam and asectioning line orthogonal to sectioning lines 84, 87 and 88 is providedin a light receiving area 82 for sub-optical beam to thereby divide eachof the light receiving areas into 8 divisional areas in total.Structurally, like the second embodiment of the invention shown in FIG.6, tracking control signal detection can be executed by a trackingcontrol signal detection circuit 500 and besides, like the fourthembodiment shown in FIG. 10, focus control signal detection can also beexecuted by a focus control signal detection circuit 501. The contentsof tracking control detection scheme and focus control signal detectionscheme has already been detailed in connection with other embodimentsand so, for avoidance of prolixity, will not be detailed here.Advantageously, with the construction of the present embodiment, foreither the focus control signal or the tracking control signal, a signalbased on the light receiving plane structure of the invention and asignal based on the conventional light receiving plane structure canselectively be switched and detected.

Incidentally, the optical pickup using the present invention is notlimited structurally to the optical system configuration or lightreceiving plane structure explained in the foregoing embodiments but mayhave any optical system configuration or light receiving plane structureso long as the optical information reproduction apparatus is mountedwith the optical pickup adopting as the tracking control signaldetection scheme a detection scheme corresponding to the DPP scheme orthe DPP scheme.

Reverting to FIG. 9, an optical information recording/reproductionapparatus mounted with the optical pickup according to the first tofifth embodiments is schematically illustrated in block diagram form. Asshown, the apparatus comprises an optical disc 900, a laser lightingcircuit 910, an optical pickup 920, a spindle motor 930, a spindle motordrive circuit 940, an access control circuit 950, an actuator drivecircuit 960, a servo signal generation circuit 970, an informationsignal reproduction circuit 980, an information signal recording circuit990 and a control circuit 9000. Responsive to an output from the opticalpickup 920, the control circuit 9000, servo signal generation circuit970 and actuator drive circuit 960 control an actuator. By using theoutput from the optical pickup in the present embodiment for actuatorcontrol, stable and highly accurate information recording andinformation reproduction can be assured.

In the course of reproducing an information signal from an optical dischaving a recording layer of multi-layer structure or recording theinformation signal in the optical disc, by using the means described asabove, the degradation in quality of tracking control signal due tointerference of an unwanted optical beam generated from a recordinglayer other than a reproduction or recording target layer with anessential signal beam can be cured sufficiently and a stable and highlyprecise tracking control signal can be detected.

Embodiment 6

Referring to FIG. 12, an example of the optical pickup according to asixth embodiment of the invention is configured as schematicallyillustrated therein.

A laser beam emitted from a laser light source 1 lands on a diffractiongrating 2 representing a beam splitting element so as to be split into amain optical beam due to a 0-th diffracted beam and two sub-opticalbeams due to positive and negative first order diffracted beams.Respective optical beams are changed in their traveling directions by apolarization beam splitter 11 and caused to go through a collimate lens4 driven by a stepping motor 12 to correct an incident optical beam forits spherical aberration, an optical element 13 having a diffractionregion for diffracting part of the main optical beam and the sub-opticalbeams and a ¼ wavelength plate 14 for giving mutually orthogonalpolarized components a phase difference of 90°, finally reaching anobjective lens 5 by which they are focused independently on apredetermined recording layer in an optical disc 10. Reflection opticalbeams of the respective focused light spots from the optical disctransmit through the objective lens 5 and reach a photodetector 8 viathe ¼ wavelength plate 14, optical element 13, collimate lens 4,polarization beam splitter 11 and detection lens 7.

Preferably, the objective lens 5, ¼ wavelength plate 14 and opticalelement 13 are mounted in an actuator 6 for driving them inpredetermined directions. A tracking control signal to be describedlater is fed back to the actuator to control the position of theobjective lens to thereby execute tracking control. As the sphericalaberration correction means, a liquid crystal device may be used.

The photodetector 8 detects the tracking control signal through the DPPscheme. The DPP scheme will be described below in brief.

An example of the DPP detection scheme will be described by makingreference to a conventional example of photodetector schematically shownin FIG. 13. Arranged in the photodetector 8 are a light receiving area80 on which a focused light spot 60 of main optical beam reflected fromthe optical disc is incident and light receiving areas 81 and 82 onwhich focused light spots 61 and 62 for sub-optical beams reflected fromthe optical disc are incident. Of these areas, the light receiving area80 for main optical beam has a light receiving plane which is quarteredby two substantially mutually perpendicular sectioning lines whereaseach of the light receiving areas 81 and 82 for sub-optical beams has alight receiving plane halved by a sectioning line 83 or 84 which issubstantially vertical to a direction corresponding to the radialdirection of the optical disc. Further, in FIG. 13, the directioncorresponding, on the photodetector, to the radial direction of theoptical disc is shown by the arrow (up and down direction on the sheetof drawing). From these divisional light receiving areas, currentscorresponding to incident light intensities are generated, convertedindependently of each other by means of current/voltage conversionamplifiers 201 to 208 and thereafter subjected to subtraction by meansof subtractors 25 and 28, so that a push-pull signal of main opticalbeam 60 (hereinafter this signal is called a main PP signal forsimplicity of explanation) and a push-pull signal as a result ofaddition of optical beams 61 and 61 (hereinafter this signal is called asub-PP signal for simplicity of explanation) can be outputted.

The main optical beam irradiates the optical disc while being spaced by½ track from the respective sub-optical beams, with the two sub-opticalbeams being irradiated while being spaced apart from each other by 1track. Accordingly, the main PP signal is outputted having a phasedifference of 180° in relation to each of the sub-PP signals. Therefore,by amplifying the two types of PP signals with suitable mu-factors K1and K2 by means of amplifiers 212 and 213, respectively, and thensubtracting them at a subtractor 214, an unwanted DC component and anin-phase disturbance component which are contained in both the main PPsignal and the sub-PP signals can be eliminated and an excellenttracking control signal can be obtained.

In this manner, in the DPP scheme, an offset of the tracking controlsignal caused concomitantly with a tracking displacement of theobjective lens can be eliminated with the simplified optical systemconfiguration and the tracking control signal of high quality can bedetected stably.

To add, in the objective lens position control in the optical pickupapparatus, not only the tracking position control but also the focusposition control which is position control along the optical axis iscarried out concurrently. As the control signal detection scheme usedfor the focus position control, the astigmatism scheme is used widely ingeneral. Like the tracking control, the focus control signal can bedetected by applying a predetermined arithmetic operation process to thedetection signal from each of the light receiving planes of thephotodetector shown in FIG. 13.

As will be seen from the above, because of its advantage, the DPP schemeprovides for the widely used detection scheme. But when the trackingcontrol signal detection mean based on the DPP scheme is used for theoptical pickup apparatus or optical information recording/reproductionapparatus for reproducing/recording the optical disk having a recordinglayer of multi-layer structure, new problems as below arise.

In reproducing/recording the multi-layer optical disc, the individualoptical beams are focused on one of recording layers which is a targetof signal reproduction/recording (the recording layer will hereinafterbe called a target layer) and reflection optical beams from the targetrecording layer are detected. In this phase, part of the quantity oflight is not reflected by the target layer but is reflected by arecording layer other than the target layer (this recording layer willhereinafter referred to as a different layer). The optical beam from thedifferent layer traces an optical path substantially identical to thatof a signal optical beam from the target layer and lands on each lightreceiving plane in the photodetector, resulting in an unwanted opticalbeam which prevents accurate detection of the signal optical beam.

This unwanted optical beam interferes with the essential signal opticalbeam on the light receiving plane, causing an interference fringe.Bright and dark stripes of the interference fringe disturb the lightquantity balance on each light receiving plane, giving rise to anunwanted interlayer cross stalk component which affects the outputsignal from each light receiving plane.

This phenomenon will be described specifically by way of example of anoptical disc 10 having two recording layers (interlayer distance δ) 100and 101 as shown in FIGS. 14A and 14B.

An optical path of an optical beam incident on the optical disc ofmulti-layer structure is schematically illustrated in FIGS. 14A and 14B,showing a state in which a main optical beam 50 and sub-optical beams 51and 52 (not shown) are focused from below on the sheet of drawing on theoptical disc 10 having two recording layers 100 and 101 on one side.

Illustrated in FIG. 14A is an instance where the individual opticalbeams are focused on the recording layer 100 (in the case of the targetlayer being recording layer 100). In this case, part of the quantity oflight of the optical beam focused on the target layer transmits throughthe target layer and is reflected at a recording layer 101 ahead of thelayer 100 to cause an unwanted optical beam 53.

Illustrated in FIG. 14B is an instance where conversely to the case ofFIG. 14A, the recording layer 101 is a target layer. In this case, theoptical beam once transmits through the front recording layer 100 andthereafter focused on the recording layer 101. In this process, however,part of the quantity of light is reflected at the recording layer 100,resulting in an unwanted optical beam 54.

Any of the unwanted optical beams 53 and 54 traces an optical pathsubstantially identical to that of the essential signal optical beam toreach the photodetector. But since the focal point of each of theunwanted optical beams 53 and 54 differs from that of the essentialoptical beam 50, the spot size of each unwanted optical beam largelydiffers from that of the essential signal optical beam on thephotodetector surface. Thus, on each of the light receiving planes, partof the unwanted optical beam overlaps the signal optical beam, causingan interference. Then, bright and dark portions of an interferencefringe disturb the balance of the quantity of light detected from eachphotodetector plane and a resulting unwanted interlayer cross talkaffects the output signal.

Especially, the sub-PP signal used for tracking control signal detectionbased on the DPP scheme has, in general, a signal intensity less thanthat of the main PP signal. Therefore, the interlayer cross talk greatlyaffects the sub-PP signal. As a result, a large waveform distortion andfluctuation are generated in the tracking control signal detectedthrough the DPP scheme and the signal quality is degraded.

Under the circumstances, in Patent Document 2, the interlayer cross talkis suppressed by using the optical element 13 provided with adiffraction area for diffracting part of the main optical beam and thesub-optical beam. This diffraction area of optical element 13 may be,for example, a diffraction grating or a polarization/diffractiongrating. In case the polarization/diffraction grating is used for thediffraction area, this optical element functions to diffract only anoptical beam reflected by the optical disc, having no influence upon thespot shape on the optical disc. An example of diffraction area 17 theoptical element 13 has is illustrated in FIG. 15. The shape of thediffraction area 17 may be changed in compliance with the shape of thephotodetector. Schematically illustrated in FIG. 16A is a lightintensity distribution on the photodetector plane when the FIG. 12optical pickup apparatus mounted with the optical element 13 takes therecording layer 100 as a target layer. Schematically illustrated in FIG.16B is a light intensity distribution on the photodetector plane whenthe FIG. 12 optical pickup apparatus mounted with the optical element 13takes the recording layer 101 as a target layer. By dint of thediffraction area of optical element 13, a dark portion 290 without lightquantity is generated in an unwanted optical beam 53. This restrains theunwanted optical beam from reaching the detector. Accordingly, theunwanted optical beam can be restrained from interfering with a signaloptical beam on the photodetector and the deterioration in a trackingcontrol signal can be reduced. By dint of the diffraction area ofoptical element 13, a diffracted light spot 291 of the diffractedunwanted optical beam is irradiated on the outside of the photodetector.With the ¼ wavelength plate 14 and optical element 13 mounted to theactuator 6, movement of the unwanted optical beam dark portion 290 onphotodetector plane concomitant with an objective lens shift can besuppressed. Consequently, even in the presence of an objective lensshift, incidence of the unwanted optical beam 53 on the photodetector 8can be reduced and an increase in interlayer cross talk can besuppressed. Similarly, in the main optical beam and sub-optical beams,dark portions 287, 288 and 289 without light quantity are formed by theoptical element 13 and their diffracted light spots 292, 293 and 294 areirradiated on the outside of the photodetector area. The spectral ratioof diffraction area 17 can be set in many ways. Accordingly, thequantity of light of the dark portions 287, 288, 289 and 290 can beadjusted in many ways. A photodetector 8 can be provided newly to detectthe quantity of light of main optical diffraction spot 292 generated bythe optical element 13 and then, a detected signal can be added to anRF-SUM signal obtained from the main optical beam receiving plane 80. Inthis manner, a more excellent jitter value, for example, can beobtained.

In the study of geometrical optics, with the optical element 13provided, an unwanted optical beam does not seem to be incident on thephotodetector. But, waveform distortion and fluctuation of trackingcontrol signal is still generated owing to an interlayer cross talk,making it difficult to detect a highly accurate and stable trackingcontrol signal.

Then, the present inventors have studied, from the standpoint of waveoptics, the degree of an influence the interference of an unwantedoptical beam with a signal optical beam has upon a sub-PP to find that,of unbalance of light quantity due to the interference, an unbalance oflight quantity generated on and near the sectioning lines 83 and 84 inthe sub-optical beam receiving planes 61 and 62 in FIG. 13,respectively, most adversely affects the quality of the sub-PP signal.

Accordingly, a photodetector representing a main part of a sixthembodiment is structured as illustrated in FIG. 17. The optical systemconfiguration of the optical pickup apparatus in the present embodimentcan be similar to that of the optical pickup shown in, for example, FIG.12.

The light receiving plane pattern of photodetector 8 in the presentembodiment features that on and near the central sectioning line 83 oflight receiving plane 81 for sub-optical beam and on and near thecentral sectioning line 84 of light receiving plane 82 for sub-opticalbeam, strip light shielding zone or dead zone 73 and 74 are provided,respectively, having a side width W, set to a dimension to be describedlater, in a direction corresponding to the radial direction of theoptical disc.

A light receiving plane 80 for main optical beam is quartered intodivisional areas 80 a, 80 b, 80 c and 80 d as shown in FIG. 17 and lightquantity signals obtained from the respective divisional areas are A, B,C and D. Further, the respective light receiving planes for sub-opticalbeams 81 and 82 are halved into divisional areas 31 a and 31 b and 32 aand 32 b as shown in FIG. 17, respectively, and light quantity signalsobtained from the respective divisional areas are I and J and K and L,respectively. The focus control signal and tracking control signal inthe present embodiment are exemplified hereunder. The focus controlsignal based on the astigmatism scheme is obtained by operating equation(1):FES:(A+C)−(B+D)  (1)The scheme for detection of the focus control signal is not limited tothe above astigmatism scheme but may use another scheme, for example, aknife edge method.

The RF signal can be obtained by operating equation (2):RF-SUM:A+B+C+D  (2)

The tracking control signal based on the DPP scheme can be obtained byoperating equation (3):TES(DPP):[(A+B)−(C+D)]−k2[(I−J)+(K−L)]  (3)and the tracking control signal based on the DPD scheme can be generatedby comparing phases of two signals, each indicated by equation (4), bymeans of the phase comparator 268:TES(DPD):(A+C),(B+D)  (4)

The light shielding zone can be realized by covering the light receivingplane with a medium having a light transmission factor of nearly zero,for example, aluminum to block incidence of optical beam on the lightreceiving plane. The light shielding zone is not limited to the mediumsuch as aluminum as above exhibiting a transmission factor ofsubstantially zero to all wavelength band of light but may be a lightshielding zone of a wavelength selectable medium, for example,exhibiting a transmission factor of substantially zero to a specifiedwavelength band. Further, the dead zone can be realized by removing, forexample, a predetermined part of light receiving plane so that even withthe optical beam being incident, no signal current may be generated.

The width W of the minor side the light shielding zone or dead zone hascan be set within a range of about 20% to 40% of the diameter each ofthe focused spots 61 and 62 of sub-optical beams incident on the lightreceiving areas 31 a and 31 b and the light receiving areas 32 a and 32b has in order to eliminate an interlayer cross talk most efficiently.More preferably, since the diameter the focused spot of sub-optical beamincident on the light receiving plane is in most general set to about100 μm, the width W is set within a range of about 20 μm to 40 μm. Thelight shielding zone or dead zone may not always take the form of astrip.

A structure to be described below will substitute for the aforementionedlight shielding zone or dead zone. Sectioning lines 95 and 96substantially parallel to the central sectioning line 83 are newlyprovided above and below the line 83 in the light receiving area 81 forsub-optical beam and sectioning lines 97 and 98 substantially parallelto the central sectioning line 84 are newly provided above and below theline 84 in the light receiving area 82 for sub-optical beam ofphotodetector, so that each of the light receiving areas 81 and 82 canbe divided into four light receiving planes. These new divisional lightreceiving planes of the light receiving area of sub-optical beam lightreceiving plane 81 are sequentially designated by 81 a, 81 b, 81 c and81 d. Similarly, the divisional light receiving planes of thesub-optical beam light receiving plane 82 are sequentially designated by82 a, 82 b, 82 c and 82 d. The spacing M between the newly providedsectioning lines 95 and 96 and the spacing M between the sectioninglines 97 and 98 are each dimensionally comparable to the width W of thelight shielding zone or dead zone has in the first embodiment shown inFIG. 1. With this structure, the individual light receiving planesdeliver signals via current/voltage conversion amplifiers and then,signals from the light receiving planes 81 a and 81 d are subtractedfrom each other and signals from the light receiving planes 82 a and 82d are subtracted from each other, resulting signals obtained by thesesubtractions being added together to provide a sub-PP signal which iscomparable to the sub-PP signal obtained from the photodetector of FIG.17.

On the other hand, signals from the light receiving planes 81 a and 81 bare added to provide a sum signal, signals from the light receivingplanes 81 d and 81 c are added to provide a sum signal, signals from thelight receiving planes 82 a and 82 b are added to provide a sum signaland signals from the light receiving planes 82 d and 82 c are added toprovide a sum signal, these sum signals being processed through anoperation similar to that described above to provide a sub-PP signalwhich is comparable to a sub-PP signal obtained from the conventionalphotodetector shown in FIG. 13. Then, a predetermined switching means isused to select the use of only the output signals from the lightreceiving planes 81 a, 81 d, 82 a and 82 d for generation of the sub-PPsignal or the use of sum signals of output signals from the lightreceiving planes 81 a, 81 d, 82 a and 82 d and output signals from thelight receiving planes 81 b, 81 c, 82 b and 82 c as well, therebyensuring that a structure having the functions of the conventionalphotodetector and the photodetector of the present invention in commoncan be attained. In this manner, the function can be selected inaccordance with the number of recording layers of the optical discsubject to recording/reproduction and therefore the general usecapability of the optical pickup apparatus can be improved.

Turning to FIG. 18, results of simulation of leakage amounts ofinter-layer cross talk to the sub-PP signal in the present embodimentand an embodiment in the Patent Document 2 are graphically illustrated,where abscissa represents positional shift amounts of light receivingplane and ordinate represents the ratio between the sub-PP signalamplitude and the inter-layer cross talk component. It will been throughcomparison of these results that as the photodetector position shifts,the interlayer cross talk amount decreases greatly over all regions inthe present embodiment and the amount can be almost halved at a positionwhere the effect is maximized. The great reduction effect in relation tothe positional shift of the photodetector is very advantageous from theviewpoint of manufacture irregularity and temporal change.

It will be appreciated that if the ratio of diffraction area 17 providedin the optical element 13 to the optical beam effective diameterincreases, the unwanted optical beam dark portion area 290 on thephotodetector also expands and the interlayer cross talk can further bereduced. But the dark portion areas 287, 288 and 289 in the main opticalbeam and sub-optical beams also expand similarly and the jitter valueand PP signal are degraded. To avoid this inconvenience, in the presentembodiment, the light shielding zones or dead zones 73 and 74 areprovided to reduce the interlayer cross talk to a great extent. Thus,with the width S of the side of the dark part areas 287, 288 or 289 in adirection corresponding to the radial direction of the optical discbeing equal or slightly smaller than the width W of the light shieldingzone or dead zone, the effect of sufficiently reducing the interlayercross talk can be attained. Accordingly, the degradation in jitter valueattributable to the dark portion area 287,288 or 289 can be suppressed.In addition, since the main PP and sub-PP signals are principallygenerated by changes in light quantity at the respective light spotouter peripheral edges, the dark portion area 287, 288 or 289 existingat the optical beam central portion hardly affects the PP signal per se.

Similarly, since the main PP and sub-PP signals are principallygenerated by changes in light quantity at the respective light spotouter peripheral edges, the light shielding zone or dead zone providedat the photodetector central portion hardly affects the PP signal perse.

Namely, in the present embodiment, by using an optical pickup apparatuscomprising an optical element having a diffraction area for diffractingpart of the main optical beam and sub-optical beams reflected at theoptical disc and a photodetector including a light receiving plane formain optical beam on which the main optical beam is incident and lightreceiving planes for sub-optical beams on which the sub-optical beamsare incident, each of the light receiving planes for sub-optical beamsbeing halved by at least one sectioning line substantially vertical to adirection corresponding to the radial direction of the optical disc andbeing provided with a strip light shielding zone of a predeterminedwidth formed on and near the sectioning line for blocking light or adead zone formed by removing the light receiving plane at that portion,the tracking control signal based on the DPP scheme can be detectedhighly accurately and stably even in the optical disc having a recordinglayer of multi-layer structure.

Embodiment 7

Next, a seventh embodiment will be described with reference to FIGS. 19Aand 19B. In the present embodiment, an optical pickup apparatus isprovided which can assure detection of an excellent DPP signal even whenthe objective lens shifts while maintaining the interlayer cross talksuppression effect the sixth embodiment attains. The optical systemconfiguration of the optical pickup apparatus can be identical to thatof the optical pickup apparatus shown in, for example, FIG. 12 with theexception that the light receiving plane pattern in a photodetector 8differs from that in FIG. 12. Illustrated in FIGS. 19A and 19B is thephotodetector 8 representing a main part of the seventh embodiment. Inthe present embodiment, sectioning lines 91 and 92 which aresubstantially parallel to a central sectioning line 90 of lightreceiving area for main optical beam are newly formed above and belowthe line 90, so that the light receiving plane for main optical beam isdivided into 8 light receiving divisional areas. Also illustrated inFIG. 19A in association with the photodetector is an arithmeticoperation circuit for performing an operation process to be describedlater to generate a tracking control signal based on the DPP scheme. InFIG. 19B, an arithmetic operation circuit for performing an operationprocess to be described later to generate a focus error signal and atracking control signal based on the DPD scheme is illustrated inassociation with the photodetector.

Factors of degrading the DPP signal at the time of objective lens shiftwill be described with reference to FIGS. 17 and 20.

When the objective lens shifts, the light intensity of signal opticalbeam is distributed on the photodetector schematically illustrated inFIG. 20. As the objective lens shifts in the radial direction of theoptical disc, the main optical beam spot and the sub-optical beam spotsmove in a direction corresponding to the optical disc radial direction(up and down direction in the figure) on the photodetector plane. Incase the objective lens shift amount causes the spot position as shownin FIG. 17 (in the case of the objective lens shift amount being small),a main optical beam dark portion 287 due to the optical element 13 is onthe sectioning line representing the boundary between main PP signaldetection divisional areas. On the other hand, since the sub-PPdetection area on the light receiving plane for sub-optical beam differsin shape from the main PP detection area, each of the sub-optical beamdark portions 288 and 289 does not exist on the boundary between thesub-signal detection divisional areas. Accordingly, near the boundary,the sub-PP signal is more sensitive to the generation amount of offsetdue to the objective lens shift amount than the main PP signal.

Next, the case shown in FIG. 20 is considered where the objective lensshift amount is large and the main optical beam spot 60 and thesub-optical beam spots 61 and 62 are positioned as shown in FIG. 20 onthe photodetector 8. This demonstrates an instance where the objectivelens shift amount is large and so the moving amounts of the spots on thedetector plane are also large. In this case, the main optical beam darkportion does not exist on the main PP detection area boundary butconversely, the sub-optical beam dark portion exists on the sub-PPdetection divisional area boundary. Accordingly, near the boundary, themain PP signal is more sensitive to the generation amount of offset dueto the objective lens shift amount than the sub-PP signal.

Thus, the visual field characteristics differ for the main PP and sub-PPover the whole area of objective lens shift and a large offset isgenerated in the tacking control signal based on the DPP scheme. Such anoffset of the tracking control signal generates de-track to make thestable and highly accurate tracking control difficult.

In the present embodiment, by dividing the main optical beam lightreceiving plane into eight divisions, an extreme degradation in thetracking control signal at the time of objective lens shift can becured. From the respective divisional light receiving planes, currentsare generated in accordance with incident light intensities, convertedindependently by current/voltage conversion amplifiers 201 to 208 andthose 270 to 273 and then subjected to an arithmetic operation processso that a focus control signal and a tracking control signal may beoutputted. The main optical beam light receiving plane 80 is dividedinto divisional areas 80 a, 80 b, 80 c, 80 d, 80 e, 80 f, 80 g and 80 has shown in FIGS. 19A and 19B and light quantity signals obtained fromthe respective divisional areas are A, B, C, D, E, F, G and H. Each ofthe sub-optical beam light receiving planes 81 and 82 is divided intodivisional areas 31 a, 31 b and 32 a, 32 b and light quantity signalsobtained from the respective divisional areas are I, J and K, L. Anexample of each of the focus control signal and tracking control signalwill be described below. The focus control signal based on theastigmatism method can be obtained by calculating equation (5):FES:[(A+E)+(C+G)]−[(B+F)+(D+H)]  (5)In the present embodiment, however, the focus control signal detectionmethod is not limited to the astigmatism method, either and anothermethod such as the knife edge method may be used.

The RF signal can be obtained by calculating equation (6):RF-SUM:A+B+C+D+E+F+G+H  (6)

The tracking control signal based on the DPP scheme can be obtained bycalculating equation (7):TES(DPP): [(A+E)+(B+F)]−(C+G)−(D+H)]−k2[(I−J)+(K−L)]  (7)and the tracking control signal based on the DPD scheme can be obtainedby comparing phases of two signals indicated by equation (8) by means ofthe phase comparator 268:TES(DPD):[(A+E)+(C+G)],[(B+F)+(D+H)]  (8)

At the time of the tracking control based on the DPP scheme, a de-trackis caused as the objective lens shifts in the sixth and seventhembodiments and the amount of de-track can be estimated as graphicallydepicted in FIG. 21. In the sixth embodiment, the objective lens shiftis accompanied by a large de-track. On the other hand, in the seventhembodiment, the objective lens shift is constantly accompanied by asmall de-track amount and it will be seen that an excellent trackingcontrol signal can be detected.

At that time, by making the spacing T between the newly providedsectioning lines 91 and 92 substantially equal to the width W of thelight shielding zone or dead zone in the sixth embodiment, the objectivelens visual field characteristics can be improved most efficiently.

In other words, in the present embodiment, by using the main opticalbeam light receiving plane which is divided into eight divisional areasby a single sectioning line substantially parallel to a directioncorresponding to the radial direction of the optical disc and by threesectioning lines substantially vertical to the radial direction (first,second and third sectioning lines), suppression of interlayer cross talksubstantially equal to that in the sixth embodiment and focus errorsignal detection can be assured and besides, by suppressing an offset ofthe tracking control signal based on the DPP scheme as the objectivelens shifts, a stable tracking control signal can be detected highlyaccurately.

Embodiment 8

Referring now to FIG. 22, an eighth embodiment will be described. FIG.22 is a schematic diagram showing a photodetector representing aprinciple part of the eighth embodiment. The optical systemconfiguration of an optical pickup apparatus in the present embodimentmay be similar to that of the optical pickup apparatus shown in, forexample, FIG. 12. The photodetector 8 shown in FIG. 22, however, has alight receiving plane pattern different from that shown in FIG. 12.

The main optical beam light receiving plane is first divided into threedivisions by two sectioning lines 91 and 92 which are substantiallyvertical to a direction corresponding to the radial direction of theoptical disc and excepting a central division area 80 k, the remainingtwo of the three divisional areas the light receiving plane has arehalved, respectively, by sectioning lines 93 and 94 substantiallyparallel to the direction corresponding to the radial direction of theoptical disc, so that the light receiving plane is divided into 5divisions in total.

In the present embodiment, by dividing the main optical beam lightreceiving plane into the 5 divisional areas, the degradation in thetracking control signal at the time of the objective lens shift can becured sufficiently. Signals are outputted from the respective divisionalareas in accordance with light intensities incident thereon andsubjected to an arithmetic operation process to be described later sothat a focus control signal and a tracking control signal may beoutputted. The main optical beam light receiving plane 80 is dividedinto divisional areas 80 a, 80 b, 80 c, 80 d and 80 k as shown in FIG.22 and light quantity signals obtained from the respective divisionalareas are A, B, C, D and M. The respective sub-optical beam lightreceiving areas are divided into divisional areas 31 a and 31 b anddivisional areas 32 a and 32 b and light quantity signals obtained fromthe respective divisional areas are I, J and K, L. Examples of focuscontrol signal and tracking control signal will be described below. Thefocus control signal based on the astigmatism method can be obtained bycalculating equation (9):FES:(A+C)−(B+D)  (9)

The focus control signal detection method in the present embodiment isnot limited to the astigmatism method but may be based on another methodsuch as knife edge method.

The RF signal can be obtained by calculating equation (10):RF-SUM:A+B+C+D+M  (10)The tracking control signal based on the DPP scheme can be obtained bycalculating equation (11):TES(DPP):[(A+B)−(C+D)]−k2[I−J)+(K−L)]  (11)

In the present embodiment, interlayer cross talk suppressive effect andobjective lens visual field characteristic improving effect comparableto those in the seventh embodiment can be obtained. Further, because themain optical beam light receiving plane is divided by a less number thanthe seventh embodiment, the present embodiment is advantageous over theseventh embodiment in that the number of amplifiers can be reduced and alow noise photodetector can be provided. By setting the spacing Tbetween the sectioning lines 91 and 92 on the main optical beam lightreceiving plane substantially equally to the minor side width W of thelight shielding zone or dead zone, the objective lens visual fieldcharacteristics can be improved most efficiently.

Namely, in the present embodiment, by using the main optical beam lightreceiving plane which is divided into three divisions by two sectioninglines substantially vertical to a direction corresponding to the radialdirection of the optical disc (first and third sectioning lines), witheach of the remaining two divisional areas excluding the centraldivisional area of the three divisional areas halved by a singlesectioning line substantially parallel to the radial direction of theoptical disc so that the main optical beam light receiving plane can bedivided into five divisions in total, the interlayer cross talksuppression effect and the objective lens visual field characteristicimproving effect comparable to those in the seventh embodiment can beobtained and besides, a photodetector of lower noise than thephotodetector of the seventh embodiment can be provided.

Embodiment 9

Next, a ninth embodiment will be described with reference to FIG. 23.FIG. 23 is a schematic diagram showing a photodetector representing aprinciple part of the ninth embodiment. The optical system configurationof an optical pickup apparatus in the present embodiment may be similarto that of the optical pickup apparatus shown in, for example, FIG. 12.The photodetector 8 shown in FIG. 23, however, has a light receivingplane pattern different from that shown in FIG. 12. The main opticalbeam light receiving plane is divided into six divisions by twosectioning lines 91 and 92 which are substantially vertical to adirection corresponding to the radial direction of the optical disc andby a single sectioning line substantially parallel to the directioncorresponding to the radial direction of the optical disc.

In the present embodiment, by dividing the main optical beam lightreceiving plane into the 6 divisional areas, the degradation in thetracking control signal at the time of the objective lens shift can becured drastically. Signals are outputted from the respective divisionalareas in accordance with light intensities incident thereon and then aresubjected to an arithmetic operation process to be described later sothat a focus control signal and a tracking control signal may beoutputted. The main optical beam light receiving plane 80 is dividedinto divisional areas 80 a, 80 b, 80 c, 80 d, 80 i and 80 j as shown inFIG. 23 and light quantity signals obtained from the respectivedivisional areas are A, B, C, D, N and O. The respective sub-opticalbeam light receiving areas 81 and 82 are divided by divisional areas 31a and 31 b and divisional areas 32 a and 32 b and light quantity signalsobtained from the respective divisional areas are designated by I, J andK, L. Examples of focus control signal and tracking control signal willbe described below. The focus control signal based on the astigmatismmethod can be obtained by calculating equation (12):FES:(A+C)−(B+D)  (12)The focus control signal detection method in the present embodiment isnot limited to the astigmatism method but may be based on another methodsuch as knife edge method.

The RF signal can be obtained by calculating equation (13):RF-SUM:A+B+C+D+N+O  (13)

The tracking control signal based on the DPP scheme can be obtained bycalculating equation (14):TES(DPP):[(A+B)−(C+D)]−k2[(I−J)+(K−L)]  (14)The tracking control signal based on the DPD scheme can be obtained byadding output signals N and O from current/voltage conversion amplifiers275 and 276 pursuant to equation (15) or (16):TES(DPD):(A+C+N),(B+D+O)  (15)TES(DPD):(A+C+O),(B+D+N)  (16)

One of the operations as above is executed selectively depending on apositional shift of the photodetector in a direction corresponding tothe radial direction of the optical disc and by comparing phases of therespective signals by means of the phase comparator 268, so that atracking control signal based on the DPD scheme of lower noise than inthe seventh embodiment and of higher accuracy than in the eighthembodiment can be generated to advantage.

Advantageously, in the present embodiment, the interlayer cross talksuppressive effect and objective lens visual field characteristicimproving effect comparable to those in the eighth embodiment can beobtained and besides the DPD signal detection can be done moreaccurately than in the eighth embodiment. By setting the spacing Tbetween the sectioning lines 91 and 92 on the main optical beam lightreceiving plane substantially equally to the minor side width W of thelight shielding zone or dead zone, the objective lens visual fieldcharacteristics can be improved most efficiently.

Namely, in the present embodiment, by using the main optical beam lightreceiving plane which is divided into six divisions by two sectioninglines substantially vertical to a direction corresponding to the radialdirection of the optical disc (first and third sectioning lines) and bya single sectioning line substantially parallel to the directioncorresponding to the radial direction of the optical disc, theinterlayer cross talk suppression effect, objective lens visual fieldcharacteristic improving effect and noise level comparable to those inthe eighth embodiment can be obtained and besides, the accuracy ofdetection of a DPD signal can be improved to advantage.

Embodiment 10

Next, a tenth embodiment will be described with reference to FIG. 24.The optical system configuration of an optical pickup apparatus in thepresent embodiment may be similar to that of the optical pickupapparatus shown in, for example, FIG. 12. In the present embodiment,mount or dismount of the optical element 13 does not matter.

FIG. 24 is a schematic diagram showing the shape of a diffractiongrating 2 representing a principal part of the present embodiment. Thediffraction grating is divided into three divisional areas by at leasttwo sectioning lines substantially vertical to a direction correspondingto the radial direction of the optical disc and of the three areas,excepting a central area 22, only areas 21 and 23 have each gratinggrooves extending in a direction substantially vertical to the twosectioning lines at a predetermined period, thus providing diffractiongrating. Then, the central area 22 may be transparent and flat.

FIG. 25 shows a photodetector 8 used in the present embodiment andindividual optical beam spots on the photodetector. When a laser beamlands on the diffraction grating of this embodiment, a sub-optical beamis generated which does not substantially have light quantity at thecentral strip zone because only the central area does not have thefunction to diffract.

The present inventors have examined the trisected diffraction gratingand found it having larger suppression effect than that obtained whenthe general diffraction grating without divisional areas is applied.

Also, in the present embodiment, the optical element 13 and ¼ wavelengthplate 14 are not mounted in the actuator 6. Accordingly, the actuatorcan be reduced in weight and excellent servo characteristics can beobtained to advantage. Because of disuse of the optical element 13, nodark portion 287 exists in the main optical beam spot and a moreexcellent jitter value than that in embodiment 6 can be obtained.

Namely, in the present embodiment, by using, as a beam splittingelement, the diffraction grating structure which is formed with gratinggrooves arranged at a predetermined period in a direction correspondingto a direction substantially parallel to the radial direction of theoptical disc or the diffraction grating structure which is divided, byat least two sectioning lines substantially vertical to the directioncorresponding to the radial direction of the optical disc, into threeareas of which only right and left two areas excepting a central areaare each formed with the grating grooves arranged at a predeterminedperiod and extending in a direction substantially vertical to the twosectioning lines, the actuator can be reduced in weight, excellent servocharacteristics can be obtained and besides, because of elimination of adark portion due to the optical element 13 from the main optical beamspot, an excellent jitter value can be obtained to advantage.

Embodiment 11

Referring to FIG. 26, an optical disc apparatus mounting the opticalpickup apparatus according to the sixth to tenth embodiments isillustrated in schematic diagram form. The optical disc apparatuscomprises an optical disc 900′, a laser lighting circuit 910′, anoptical pickup apparatus 920′, a spindle motor 930′, a spindle motordrive circuit 940′, an access control circuit 950′, an actuator drivecircuit 960′, a servo signal generation circuit 970′, an informationsignal reproduction circuit 980′, an information signal recordingcircuit 990′ and a control circuit 9000′. Responsive to an output fromthe optical pickup 920′, the control circuit 9000′, servo signalgeneration circuit 970′ and actuator drive circuit 960′ control anactuator. By using, for actuator control, the output from the opticalpickup apparatus according to the present invention, stable and highlyaccurate information recording and information reproduction can beattained.

Obviously, the optical pickup apparatus using the present invention isnot limited to the optical system shown in FIG. 12 and the opticalsystem configuration or light receiving plane structure explained inconnection with the embodiments.

Through the use of the components as above, when reproducing aninformation signal from an optical disc having a recording layer ofmulti-layer structure or recording an information signal on therecording layer, the degradation in quality of a tracking control signalcaused by the interference of an unwanted optical beam stemming from arecording layer other than a target layer for reproduction or recordingwith an essential signal optical beam can be cured sufficiently and astable and highly accurate tracking control signal can be detected.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. An optical pickup comprising: a semiconductor laser light source; abeam splitting element having the function to split a laser optical beamemitted from said semiconductor laser light source into a main opticalbeam and sub-optical beams; an objective lens for focusing the mainoptical beam and each of the sub-optical beams on a predeterminedrecording layer provided in an optical information recording medium; anda photodetector for independently receiving reflections of the mainoptical beam and sub-optical beams from said recording layer to detectpredetermined signals, wherein the photodetector includes a lightreceiving plane for main optical beam on which the main optical beam isincident and which is divided into four divisional areas by two mutuallysubstantially orthogonal sectioning lines; and light receiving planesfor sub-optical beams on which the sub-optical beams are incident andeach of which is divided into four divisional areas by two mutuallysubstantially orthogonal sectioning lines and being capable of detectinga focus control signal based on an astigmatism scheme wherein the lightreceiving planes for sub-optical beams each further includes astrip-shaped light shielding zone provided on and near any of the twosectioning lines provided for the light receiving plane for sub-opticalbeams and having a predetermined width and substantially completelyblocking the incident light, and wherein the light shielding zone has awavelength-selectability which exhibits a different transmission factorin accordance a wavelength band of the laser optical beam such that thetransmission factor becomes substantially zero for a predeterminedwavelength band of the laser optical beam.
 2. The optical pickupaccording to claim 1, wherein the light shielding zone provided in eachof the light receiving planes for sub-optical beams has a minor sidewidth amounting to a value within a range of 20% to 40% of the diameterof a focused light spot of the sub-optical beam irradiated on each ofthe light receiving areas for sub-optical beams.
 3. An opticalinformation reproduction apparatus, comprising: an optical pickupaccording to claim 2, and a function of detecting a tracking controlsignal based on a differential push-pull system by independentlydetecting predetermined signals based on the push-pull system fromrespective ones of the light receiving plane for main optical beam andlight receiving planes for sub-optical beams and subjecting detectedsignals to a predetermined arithmetic processing.
 4. The optical pickupapparatus according to claim 1, wherein the light shielding zoneprovided in each of the light receiving planes for sub-optical beams hasa minor side width amounting to a value within a range of 20 μm to 40μm.
 5. An optical information reproduction apparatus, comprising: anoptical pickup according to claim 1, and a function of detecting atracking control signal based on a differential push-pull system byindependently detecting predetermined signals based on the push-pullsystem from respective ones of a light receiving plane for main opticalbeam and light receiving planes for sub-optical beams and subjectingdetected signals to a predetermined arithmetic processing.
 6. An opticalpickup comprising: a semiconductor laser light source; a beam splittingelement having the function to split a laser optical beam emitted fromsaid semiconductor laser light source into a main optical beam andsub-optical beam; an objective lens for focusing the main optical beamand each of the sub-optical beams on a predetermined recording layerprovided in an optical information recording medium; and a photodetectorfor independently receiving reflections of the main optical beam andsub-optical beams from said recording layer to detect predeterminedsignals, wherein the photodetector includes a light receiving plane formain optical beam on which the main optical beam is incident and whichis divided into four divisional areas by two mutually substantiallyorthogonal sectioning lines; and light receiving planes for sub-opticalbeams on which the sub-optical beams are incident and each of which isdivided into four divisional areas by two mutually substantiallyorthogonal sectioning lines and being capable of detecting a focuscontrol signal based on an astigmatism scheme, wherein a strip-shapedregion of a predetermined width is provided on and near any of the twosectioning lines provided for each of the light receiving planes forsub-optical beams, the strip-shaped region being constituted of a deadzone having substantially zero current or voltage-output sensitivity tothe intensity of the incident light, and wherein the dead zone has awavelength-selectability which exhibits a different transmission factorin accordance with a wavelength of the laser optical beam such that thetransmission factor becomes substantially zero for a predeterminedwavelength band of the laser optical beam.
 7. The optical pickupaccording to claim 6, wherein the dead zone provided in each of thelight receiving planes for sub-optical beams has a minor side widthamounting to a value within a range of 20% to 40% of the diameter of afocused light spot of the sub-optical beam irradiated on each of thelight receiving areas for sub-optical beams.
 8. The optical pickupapparatus according to claim 6, wherein the dead zone provided in eachof the light receiving planes for sub-optical beams has a minor sidewidth amounting to a value within a range of 20 μm to 40 μm.